Photosensors or photodetectors are sensors that detect light or other electromagnetic energy. There are several varieties of photosensors or photodetectors, many of which are manufactured using conventional CMOS technologies. For example, photosensors or photodetectors can be active receivers commonly used in photonic integrated circuit (PIC) transceivers. These types of transceivers have emerged as an alternative to transceivers that use discrete opto-electronic components.
Many types of photosensors or photodetectors implement CMOS integrated nanophotonics circuits. These nanophotonics circuits include crystalline materials like germanium or III-V compounds, which are desirable for use as the active element in photodetector components. This is due to their high quantum efficiency. In the manufacturing process, the crystalline materials are encapsulated in order to protect the crystalline structure from other manufacturing processes.
Using rapid melt growth, amorphous or polycrystalline films (e.g., germanium or III-V compounds) can be deposited at low temperatures in an amorphous or polycrystalline state, and then crystallized thermally. This technique provides for a high degree of integration flexibility. During the crystallization anneal, though, the amorphous or polycrystalline material (e.g., Ge) expands and contracts, creating stress on the encapsulation films. These stresses can be exacerbated due to the encapsulation films being formed on non-planar surfaces such as divots formed by processing of shallow trench isolation (STI) structures.
These stresses can create a breach in the encapsulation, resulting in defects that can subsequently degrade the operation of the photodetector. For example, light coupled to extrusions can result in slow diffusion of carriers to contacts, thereby limiting a 3 db bandwidth of the detector.